Processes for the isomerization of paraffinic hydrocarbons are used in the petroleum processing and petrochemical industry to increase the octane numbers of gasoline fractions and to obtain individual isomers.
In connection with the introduction of standards that limit the content of oxygenates and aromatic hydrocarbons in motor fuel, isomerizate becomes the principal source of hydrocarbons with high antiknock quality. Processes of isomerization of paraffinic hydrocarbons on heterogeneous acid catalysts are use to obtain the isomerizate. However, many isomerization catalysts promote the formation of C1-C4 gases due to the cracking reaction, which decreases the total yield of C5+ hydrocarbons.
Recently, catalysts based on zirconium oxide have been actively studied. Catalysts based on zirconium oxide, using various oxo-anions as promoters, have been considered in publications U.S. Pat. Nos. 6,180,555 B1, 6,080,904, and 7,368,626 B2. It is also known that the most active catalysts are those based on zirconium oxide, promoted by a compound of sulfur(VI) oxide, for example, U.S. Pat. Nos. 5,494,571 and 6,037,303. In order to reduce the yield of cracking by-products and to prolong the life of the catalyst, a group VIII hydrogenating metal is introduced additionally into catalysts. At the same time, there are examples of catalysts that include in their composition, besides the hydrogenating component, rare earth metals. Lanthanide-series metals, yttrium, and group VIII metals are used as an additional promoter in U.S. Pat. No. 7,022,889. Catalysts based on zirconium oxide, promoted by oxides or hydroxides of group VI, VII, and VIII metals, oxides or hydroxides of group I-B, II-B, III-A, III-B, IV-B, V-A, or VI-A metals, as well as those containing lanthanide-series metals are described in U.S. Pat. Nos. 5,310,868 and 5,214,017.
It is known that a catalyst capable of initiating the isomerization of paraffinic hydrocarbons at low temperatures (including catalysts based on sulfated zirconium oxide) has strong acid centers, where cracking reactions proceed in parallel with isomerization reactions. In case of occurrence of parallel reactions, an important characteristic of the catalyst is its selectivity with respect to the desired end products. Selectivity may be varied by means of process parameters (process pressure, hydrogen-to-hydrocarbon ratio, process temperature, space velocity) or by the introduction of promoters—platinum group metals and/or rare earth metals. It is characteristic that to increase selectivity toward the isomerization reaction it is necessary to carry out the process at increased hydrogen pressure and a low hydrogen-to-hydrocarbon mole ratio; this impairs the economic parameters of the process. On the other hand, the introduction of rare earth metal promoters results in an increased cost of the catalyst.
The publication, U.S. Pat. No. 8,153,548 B2, is the closest to the present invention. Its authors have proposed a catalyst comprised of tungstated zirconium oxide, containing a hydrogenating/dehydrogenating component from the group VIII metals and having in its composition an alkaline component from the group I metals—lithium, sodium, potassium, rubidium, and cesium. However, as is known, tungstated zirconium manifests catalytic activity at temperatures substantially exceeding those for catalysts based on sulfated zirconium, which, according to thermodynamics, decreases the selectivity of the isomerization reaction of paraffinic hydrocarbons.